Polyolefins having a multimodal molecular weight distribution (MWD), such as polyethylene, can be made into articles by extrusion molding, thermoforming, rotational molding, etc. and have advantages over typical polyolefins lacking the multimodal MWD. Polyolefins having a multimodal MWD process more easily, i.e., they can be processed at a faster throughput rate with lower energy requirements and at the same time such polymers evidence reduced melt flow perturbations and are preferred due to improved properties for applications such as high strength films.
There are several known methods of producing polyolefins having a multimodal MWD; however, each method has its own disadvantages. Polyolefins having a multimodal MWD can be made by employing two distinct and separate catalysts in the same reactor each producing a polyolefin having a different MWD; however, catalyst feed rate is difficult to control and the polymer particles produced are not uniform in size, thus, segregation of the polymer during storage and transfer can produce non-homogeneous products. A polyolefin having a bimodal MWD can also be made by sequential polymerization in two separate reactors or blending polymers of different MWD during processing; however, both of these methods increase capital cost.
U.S. Pat. No. 4,530,914, incorporated herein by reference, discloses a method of producing polyethylene having a broad molecular weight distribution and/or a multimodal MWD. The polyethylenes are obtained directly from a single polymerization process in the presence of a catalyst system comprising two or more metallocenes each having different propagation and termination rate constants, and aluminoxane.
There are certain limits to the known methods of preparing bimodal molecular weight distribution or multimodal molecular weight distribution polyolefins. At certain ratios, and in certain polymerization processes, even catalysts having two different metallocenes on aluminoxane produce polyolefins having monomodal molecular weight distributions. Even under ideal conditions if the bimodal molecular weight distribution of the polymer needs to be changed, a new catalyst needs to be made at different metallocene ratios.
It is known to prepare aluminoxane supports by saturating an organic solvent with water and reacting it with an aluminum alkyl compound. In this process, however, the water molecules tend to cluster and can over-hydrolyze the aluminum alkyl compound, forming alumina which has to be separated; wasting reactant and increasing processing steps. It is also known to produce aluminoxane by using the water of crystallization of inorganic salts such as CuSO.sub.4 .multidot.5H.sub.2 O and Al.sub.2 (SO.sub.4).sub.3 .multidot.6H.sub.2 O, to introduce water in a monomolecular form to produce the aluminoxane. In this process, however, the remaining inorganic salt must be removed from the sometimes viscous reaction medium; increasing cost and processing steps. In these known processes of preparing aluminoxane it is not disclosed to have the metallocenes present during the reaction with water.
It is an object of the present invention to produce polyolefins having a multimodal molecular weight distribution. It is an object of the present invention to produce polyolefins having varied multimodal molecular weight distributions without altering the metallocene ratio or changing catalysts. It is a further object of the present invention to produce an improved process of making polyolefins having a multimodal molecular weight distribution. It is an object of the present invention to provide an improved process of making aluminoxane support. It is a further object of the present invention to provide an improved process of producing solid dual metal polymerization catalysts. It is also an object of the present invention to provide a process whereby a polyolefin having a varied multimodal molecular weight distribution can be produced.